This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-296077, filed Oct. 19, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a method and system for suppressing a voltage fluctuation in a power system.
The voltage maintenance of an electric power system is very important from the standpoint of the supply of an electric power. In order to suppress the voltage fluctuation, it is necessary to adjust a reactive power constituting the greatest factor of a voltage fluctuation. For such fluctuation, a state condenser, shunt reactor and/or static Var compensator (SVC) are applied to a power system.
FIG. 1 is a diagram showing a structure in which the SVC is applied to a conventional power system. In FIG. 1, a power system 1 includes a power source 11, transmission lines 12, 13 and substation bus 2 and the SVC is applied to the power system 1.
This SVC comprises a voltage transformer 3 connected to the power system 1, controller 4 for allowing the inputting of a bus voltage detected by the voltage transformer 3, thyristor controlled reactor (TCR) 6 connected to the power system 1 through a down voltage transformer 5 and adapted to be controlled by a control value from the controller 4, and state condenser 7 connected to the power system 1 through the down voltage transformer 7.
The thyristor controlled reactor (TCR) 6 comprises a rector 61 and thyristors 62 and 63 connected cathode-to-anode and the state condenser 7 comprises a switch 71 and capacitor 72. The controller 4 comprises a voltage detection circuit 41, voltage reference circuit 42, adder 43, transfer function circuit 44, Q-xcex1 function generation circuit 45 and turn-on pulse generation circuit.
The operation summary of the SVC in FIG. 1 is as follows: That is, the phase control of the thyristors 62, 63 in the thyristor controlled reactor (TCR) 6 and ON-OFF control of the switch 71 in the state condenser 7 are so performed by the controller 4 as to supply a leading phase reactive power when, in the case where a voltage on the bus of the power system 1 is detected by the voltage transformer 3 in FIG. 1, the voltage on the bus is lowered and to supply a lagging phase reactive power when the voltage on the bus is raised.
Stated in more detail about the operation of the SVC, the output of the voltage transformer 3 is converted by the power detection circuit 41 to an effective value or root-mean-square value and the output of the power detection circuit is compared by the adder 43 with the output of the voltage reference circuit 42 and a deviation is input to the transfer function circuit 44. The transfer function circuit 44 is comprised of, for example, a proportional plus integral circuit and determines a reactive power operation amount of the SVC so as to allow the deviation of the input to become zero or as small as possible, that is, to allow the voltage on the bus 2 to coincide with a set value of the voltage reference circuit 42.
The output of the transfer function circuit 44 is supplied to the Q-xcex1 function generation circuit 45 to allow a conversion to a phase control angle of the thyristors 62, 63 in the thyristor controlled reactor (TCR) 6 and allows the turn-on pulse generation circuit 46 to supply a gate pulse. At the thyristor controlled reactor (TCR) 6, the output reactive power can be continuously adjusted, by the phase of the gate pulse, in a range from zero to a capacity of the reactor 61 and, through the utilization of a voltage-reactive power characteristic on the power system 1 side, eventually the voltage on the bus 2 is adjusted.
Further, in the case where the value of the phase control angle suffers such a larger voltage fluctuation as to reach an upper limit value or lower limit value, the switch 71 of the state condenser 7 is turned ON or OFF to allow a reactive power supply amount to greatly vary. This, together with the thyristor controlled reactor (TCR), provides a broader controllable range.
The response characteristic of this SVC is determined by the transfer function circuit 44 and its control constant is determined by the structure and short-circuiting capacity of the power system 1, capacity of the thyristor controlled reactor (TCR) 6, and target response speed.
However, the structure of the power system 1 varies due to various factors, such as a power system fault, inspection servicing, and start and stop of a generator constituting a power source.
When the control constant of the transfer function circuit 44 is determined with due consideration paid to a high speed response, then there are sometimes the cases where the response becomes unstable if a short-circuiting capacity of the bus 2 becomes smaller, such as in the case where the transmission line 12 in FIG. 1, for example, is separated due to the power system fault.
When, on the other hand, the control constant of the transfer function circuit 44 is determined with due consideration paid to the stability, then there occurs an inconvenience that the response speed at a usual time is slowed.
Further, in the case where a short-circuiting capacity on the power system 1 side is greater, even if a generation amount of the reactive power is adjusted by a pulse control of the thyristor controlled reactor (TCR) 6, then a reactive power operation amount is eventually greater because a fluctuation of the voltage on the bus 2 is smaller. And there is an inconvenience that the value of the phase control angle is liable to reach an upper limit value or a lower limit value.
It is accordingly the object of the present invention to provide a method and system for compensating a voltage fluctuation occurred in the transmission line, so as to suppress a voltage fluctuation in a power system.
The above-mentioned object is achieved by the following method. That is, the method applied to a power system having transmission lines and compensating a voltage fluctuation in the transmission line, comprises the steps of detecting an electric parameter, such as characteristic impedance Zo, transmission line""s voltage V and power flow P, of the transmission lines which involves a voltage fluctuation to be compensated; calculating a control parameter for adjusting the voltage on the transmission line on the basis of the electric parameter detected by the detecting step; and compensating the voltage fluctuation in the transmission line on the basis of the control parameter calculated by the calculating step.
The above-mentioned is also achieved by the following system. That is, the system applied to a power system having a plurality of transmission lines and compensating a voltage fluctuation in the transmission line comprises detecting means for detecting an electric parameter of a transmission line of the transmission lines which involves a voltage fluctuation to be compensated, calculating means for calculating a control parameter for adjusting a voltage on the transmission line on the basis of the electric parameter detected by the detecting means, and compensating means for compensating a voltage fluctuation in the transmission line on the basis of the control parameter calculated by the calculating means.
The above-mentioned object is achieved by the following apparatus. That is, the apparatus for compensating a voltage fluctuation in a transmission line comprises an adjusting device provided on a power system having a plurality of transmission line members and adjusting at least one of a reactive power and impedance, a detecting section for detecting an electric parameter of a transmission line member of the transmission line members which involves a voltage fluctuation to be compensated, a calculation section for calculating a compensation amount for adjusting a voltage of the transmission line member on the basis of the electric parameter detected by the detecting section, and a control section for generating a control signal to be given to the adjusting device on the basis of the compensation amount calculated by the calculating section.
According to the present invention, a capacitive or inductive reactance element is arranged in a series array or parallel array on the transmission line and the reactance element can be so controlled as to satisfy an equation P=V2/Zo and a voltage fluctuation in the transmission line is compensated, so that control is so made as to maintain a voltage between the ends of the transmission line constant.
Further, the transmission line as a compensation target is initially determined and it is not affected by the other fluctuation factors on the power system and the apparatus of the present invention operates safely and, if a capacity is so initially determined as to meet a maximum power flow in the transmission line, an overload is less likely be produced.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.